214 MORPHOLOGY OF GYMNOSPERMS 



that of other gymnosperms, and particularly that of Dioon edule, 

 in which there is a temporary segmentation of the entire egg. There 

 is an extensive region of abandoned proembryonic tissue, abandoned 

 in the sense that it does not enter into the structure of the embryo; 

 a suspensor development, which in this case is very massive; and at 

 the tip of the suspensor region the organization of an embryo (figs. 

 248, 249). 



Usually there are two cotyledons, but three are by no means uncom- 

 mon. In all the Japanese material investigated by Lyon two coty- 

 ledons were present except in two cases; while 15 per cent of the 

 material from another source showed three cotyledons; and we have 

 noted that material from northern Ohio sometimes shows three 

 cotyledons. The cotyledons are normally equal and entire, springing 

 apart when liberated from the seed. The former accounts of unequal 

 and lobed cotyledons, united at the tip, seem to have been drawn from 

 unusual material. The cotyledons are hypogean, as in the cycads, 

 but they produce stomata, chiefly on the adaxial surface. From 

 this Miss WiGGLESWORTH (38) Concludes that the cotyledons are 

 true foliage leaves that have become hypogean. 



Polyembryony occurs occasionally, Lyon often finding two 

 developing embryos; but one of them usually became centrally placed 

 and the other became abortive; in only one case were two mature 

 embryos seen; in one seed three developing embryos were found. 

 Cook (35) examined 200 seeds (from trees cultivated in Washington), 

 and found four of them containing twin embryos about one-third 

 as long as normal embryos; twenty-four of the seeds contained no 

 embryos. 



In germination the plumule is thrust out of the testa by the petiole- 

 like elongation and arching of the bases of the cotyledons, the portions 

 of the cotyledons that remain within the seed enlarging and function- 

 ing as haustorial organs, persisting through the first season. The 

 first two or three leaves are small and scalelike, without any develop- 

 ment of the characteristic blades (fig. 215). The stem soon stops 

 its rapid elongation for the season and puts forth at the apex a rather 

 close crown of leaves, a large terminal bud being the only prominent 

 one on the plant during the first winter. 



